Doubt License

Hello, I work in a firm in Germany, I am the new kid, well, I program microcontrollers, but now they want that I, a small GUI interfaz app make as a part of another project. The project will be to another firm supplied (only one client).

Here, they work usually with .net and c#. I do not have much interest in .net learn.
My question is: Can the firma, a license for one month buy and during this time, build the app for later delivery to the other company?

if so, how much cost a month license?

For not with the Boss speak, Is there a way to make this without pay license?
(For example, if the code is delivered in assembly with the program)(only a single client)